There has been a constant increase in the incidence of dengue in the Americas over the past 15 years. This has been caused by increased frequency of epidemic activity in most countries, as a result of increased numbers of virus serotypes circulating in the region. The change in disease ecology has resulted in the emergence of dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS) in the region, first with a major epidemic in Cuba, followed by increased occurrence of sporadic cases of DHF/DSS in many countries. The sequence of events in the Americas in the 1980's has been nearly identical to the pattern observed in Southeast Asia in the 1950's. Prospects for prevention of epidemic DHF/DSS in the American region, therefore, are not good. In the absence of Aedes aegypti eradication, the only hope for effective prevention and control is to develop more effective active surveillance for DHF/DSS and combine that with both emergency and community-based vector control programs.